Treatments for cancer impose substantial burden and morbidity but net survival benefit of different strategies is often small and virtually always uncertain. Thus, clinicians may do more harm than good if treatment is too aggressive. There are a number of management strategies for breast cancer aimed at reducing unnecessary morbidity and burden on patients with favorable disease. A key barrier to advancing these initiatives is the need for a better understanding of the challenges of individualizing cancer care The goal of this program is to improve population health by helping clinicians and their patients address the challenges of individualizing treatment of breast cancer for patients with favorable prognosis. Objective 1: To examine the challenges of individualizing treatment for women with breast cancer. Two projects will each undertake an observational study of patients newly diagnosed with breast cancer (including their attending clinicians) who were reported to the SEER registries of Georgia and New Jersey during an 18 month period to examine patient and clinician factors associated with key evaluative tests, treatments, and patient appraisal of decision quality. Project 1 will focus on challenges for surgeons and their patients with regard to locoregional therapy. Project 2 will focus on challenges for medical oncologists and their patients with regard to systemic therapy. Objective 2: To improve treatment decision quality. Project 3 will perform a randomized controlled trial of a practice based online decision tool for patients newly diagnosed with breast cancer in the Detroit and Atlanta SEER regions intended to improve decision quality. Objective 3: To accelerate the dissemination of SEER-based research findings by implementing and evaluating a tailored online portal aimed at all surgeons and medical oncologists who treated the patient samples in P1 and P2 to evaluate whether our research findings can more directly and promptly inform clinicians knowledge and attitudes about individualizing treatment. Objective 4: To advance methods in SEER population translational research focused on quality of cancer care. We propose four shared resource cores that will support program project activities, advance innovative methods in oncology population sciences, and advance team mission and long-term strategic planning.